Six Nato soldiers have been killed in southern Afghanistan after the vehicle they were travelling in was hit by a roadside bomb, a Nato spokesperson said.
At least one soldier was injured in the blast on Sunday.
Reuters reports that Canada's defence department said the six killed were Canadians.
It was the single worst incident for the multinational force this year.
Earlier in the day, one Nato soldier was killed and two injured by a similar bomb in another part of the south, a Nato spokesperson said.
About 5,000 Nato and Afghan troops are engaged in a large offensive, Operation Achilles, in Helmand province in the south, the opium heartland of the world's biggest producer.
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British and Canadian troops make up the bulk of the International Security Assistance Force (Isaf) in the south of the country.
Canada's department of national defence said on Sunday that the six soldiers killed were Canadians.
Lieutenant Morgan Bailey, in Ottawa, said: "I can confirm they are Canadian but the identities have not been released. I don't have the specific details on the incident itself but they will be coming out shortly."
The latest deaths bring the Canadian toll to 51 since Ottawa sent troops to Afghanistan in 2002. Canada's main base is in the southern city of Kandahar.
Last year saw the bloodiest fighting in Afghanistan since the Taliban were ousted from power in 2001, mainly in the southern heartland of Helmand and neighbouring Kandahar province.
Britain has said it will send another 1,400 soldiers soon - making its deployment in Afghanistan greater than in Iraq. The United States has also committed an extra 3,200 soldiers.